The OFDM (orthogonal frequency division multiplexing) transmission system has tolerance against multipath interference, so is known as a transmission system suitable for digital transmission using terrestrial waves. The OFDM transmission system is being employed as a provisional system by the Japanese Telecommunications Technology Council for its digital terrestrial television broadcast system (hereinafter referred to as the “wideband ISDB-T” system) and digital terrestrial radio system (hereinafter referred to as the “narrowband ISDB-T system”).
At the start of digital broadcasts, it can be considered that there will be a duration of coexistence with the analog broadcasts already in service. Also, it is proposed to abolish analog broadcasts after the shift to digital broadcasts has sufficiently advanced. For the duration of coexistence of analog broadcasts and digital broadcasts, it is necessary to give sufficient consideration so that the digital broadcasts do not interfere with the already existing analog broadcasts. In the wideband ISDB-T system comprised by 13 segments, as shown in FIG. 1, one segment's bandwidth (about 429 kHz) is provided as a guard band to separate upper and lower adjacent channels (so that there is no interference from an adjacent channel).
FIG. 1 shows a case where such consideration is given by providing a guard band between each two adjacent channels among a channel 1 (ch1) having a center frequency f1, ch2 having a center frequency f2, and ch3 having a center frequency f3.
However, the provision of guard bands as mentioned above means a reduction of the efficiency of use of the frequency, so is not preferred from the viewpoint of effective utilization of the frequency. Also, when the analog broadcasts are abolished and digital broadcasts are shifted to, it is desired that there be no interference between adjacent digital signals even if no guard bands are provided.